1. Field of the Invention
The present invention relates to an image-forming device, such as a laser printer, and an optical scanner provided in the image-forming device.
2. Description of the Related Art
Conventional image-forming devices, such as laser printers are commonly equipped with an optical scanner functioning to form electrostatic latent images on a photosensitive member based on image data by scanning a laser beam over the surface of the photosensitive member.
One such optical scanner disclosed in Japanese unexamined patent application publication No. 2005-62892 includes a semiconductor laser; a polygon mirror unit having a polygon mirror formed in the shape of a flat polygonal column, each side surface of which is a reflecting surface for reflecting a laser beam, and a polygon motor for driving the polygon mirror to rotate; and an scanning optical system for forming an image on the surface of the photosensitive member with the laser light deflected and scanned by the polygon mirror unit. The semiconductor laser, the polygon motor, and the scanning optical system are supported on a common support frame, while the polygon mirror is supported on a drive shaft of the polygon motor.
In recent years, there has been demand for optical scanners with increased speed and precision. To meet such demands, it is necessary to increase the rotational speed of the polygon mirror (polygon motor). However, increasing the rotational speed of the polygon mirror generates greater vibrations in the polygon mirror unit. These vibrations vibrate the scanning optical system supported on the support frame, which produce displacement in the laser beam. This displacement causes irregularities in the scanning position of the laser beam on the surface of the photosensitive member, degrading the quality of the electrostatic latent image formed thereon.
To reduce the vibrations in the scanning optical system generated by the vibrations of the polygon mirror unit, it is conceivable to increase the number of reinforcing ribs used to reinforce the support frame in order to enhance the stiffness of the support frame. However, adding more reinforcing ribs increases the complexity of the support frame structure, leading to increased costs for manufacturing the support frame.
On the other hand, Japanese Patent No. 3,137,195 proposes a technique for preventing stray light from being irradiated onto the surface of the photosensitive member by providing shielding plates between the polygon mirror and the photosensitive member for shielding the laser beam deflected by the polygon mirror at both ends in the main scanning direction.